1. Field of the Invention
The present invention relates to electrophotographic photosensitive members and more particularly to an electrophotographic photosensitive member containing a specific azo pigment.
2. Description of the Prior Art
As photosensitive members having layers containing organic pigments on a conductive layer known in the art, there are:
(i) a photosensitive member having a layer containing organic pigments dispersed in an insulating binder provided on a conductive layer, as disclosed in Japanese Patent Publication No. 1667/1977 (Electrophotographic plate);
(ii) a photosensitive member having a layer containing organic pigments dispersed in a charge transport medium, comprising a charge transport material or a combination of said material with an insulating binder (binder itself may be a charge transport material), provided on a conductive layer, as disclosed in U.S. Pat. No. 3,894,868 (Electrophotographic plate) and U.S. Pat. No. 3,870,516 (Electrophotographic imaging method);
(iii) a photosensitive member, comprising a conductive layer, charge generation layer containing organic pigments and a charge transport layer, as disclosed in U.S. Pat. No. 3,837,851 (Electrophotographic plate);
(iv) a photosensitive member, comprising organic pigments added in a charge-transfer complex, as disclosed in U.S. Pat. No. 3,775,105 (Photoconductive member); and
(v) other types of members.
While a wide variety of pigments, including phthalocyanine pigments, polycyclic quinone pigments, azo pigments, and quinacridone pigments, have been proposed for use in these photosensitive members, few of them have been actually used.
The reason is that organic photoconductive pigments are generally inferior in sensitivity and durability to inorganic ones such as Se, CdS, and ZnO.
Meanwhile, inorganic photosensitive materials have the following drawbacks:
In selenium base photosensitive members, the crystallization of photosensitive material is readily promoted by heat, moisture, dust, fingerprints, or other factors, remarkably in particular when the atmospheric temperature exceeds about 40.degree. C., resulting in deterioration of the charge bearing characteristics and appearance of white spots in the images. Although the life span of selenium base photosensitive members is said to be as long as about 30-50 thousand copies (in terms of copying capacity per life), the present situation is that many of these elements cannot attain in practice such a life span because the environmental conditions are diversified depending upon the territory and position where the copying machine is set.
Cadmium sulfide base photosensitive members, though having about the same life span as selenium base members, have poor moisture resistance, due to cadmium sulfide itself, which is difficult to overcome, and therefore require some auxiliary means, for example, such as a heater, in order to prevent the moisture absorption.
Zinc oxide base photosensitive members, because they contain a sensitizing dye typified by Rose Bengal, involve the problems of charge deterioration and light fading of the dye, and the life span thereof is at present only about 1000 copies.
Sensitivities of conventional photosensitive members, expressed in exposure quantity for halving original potential, E 1/2, are in the order of 15 lux.sec for those of unsensitized Se type, 4-8 lux.sec for those of sensitized Se type and of CdS type, and 7-12 lux.sec for those of ZnO type.
Desirable sensitivity of practical photosensitive members are of E 1/2 value up to 20 lux.sec in the case of ordinary plane paper copying machines and up to 15 lux.sec in the case of high-speed copying machine, though members of lower sensitivity may be used in certain applications.
On the other hand, various kinds of organic photoconductive polymers including poly(vinylcarbazole) in the first place and low-molecular organic photoconductive substances such as 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole have been proposed. It is difficult to use actually these materials, in spite of their superiority in lightness to the above-said inorganic photoconductive materials. The reason for the unsuccessfulness is that these organic photoconductive substances are inferior to inorganic ones in sensitivity, durability, and stability to the change of surrounding conditions.